Officials still don’t know how more than 200 people in the U.S. fell beginning in mid June, but they know why — most were infected with the cyclospora parasite.

As summer started, scientists at the U.S. Centers for Disease Control and Prevention (CDC) were notified of two people from Iowa who had developed unusual cases cyclospora infection. The Iowans had no history of international travel in the two weeks prior to their infections, which strongly indicated that their bugs were contracted stateside. Since then more infections have been reported to the CDC, totaling up to 285 people in 11 states.

CDC and state public health officials are still investigating how the people got sick, but here is what’s known so far:

Centers for Disease Control and Prevention

The cases were reported to the CDC by state health departments and physicians as cyclospora infection, but the agency generally conducts additional testing to confirm the diagnosis. So far, CDC has confirmed the presence of the parasite in samples from five of the cases.

18 people have been hospitalized.

Cyclospora is a one-celled, microscopic parasite that causes weeks or even months of severe intestinal illness associated with diarrhea and sometimes explosive bowel movements. Other symptoms include loss of appetite, cramps, nausea and fatigue. Many cases of the infection resolve on their own, but antibiotics can speed recovery and reduce symptoms. Often, they symptoms appear to go away, but relapse a few more times.

Generally, cyclospora outbreaks occur when food or water is contaminated with the parasite. The parasite is more common in tropical countries, and previous outbreaks in the U.S. originated in fresh produce that was imported from these regions.

What they don’t know at this point includes:

the source of the contamination, whether it’s contaminated produce or unsanitary irrigation methods used to grow produce

whether there is a single or multiple contaminated products that are responsible for the infections

As the investigation continues, the CDC is urging people to take precautions when eating fresh produce in particular. “The best way to prevent infection is to avoid the food causing it, but since we don’t know what that is yet, as always, people should make sure they wash any fresh food to reduce risk,” says a CDC spokesperson. The agency will update their investigation here.